The overall objective of this study is to determine the role of cellular immunity in fibroma tumor regression either as an anti-tumor cell function or an anti-virus function. Lymphocyte and macrophage cytotoxicity to fibroma tumor cells will be evaluated in adult rabbits with regressing tumors and new-born rabbits with progressive growing tumors (Cr51 release test; microcytotoxicity test) to determine if newborn tumor-bearing rabbits have a defective cell-mediated immunity. Anti-tumor cell immunity of macrophages will be related to anti-viral immunity (ability to restrict virus replication; interferon production) in the adult and newborn rabbits. The mechanism and specificity of macrophage activation will be studied in vitro by evaluating the roles of T-cells and/or cytophilic antibodies on macrophage activation. These studies will be performed using fibroma virus, myxoma virus, and vaccinia virus.